There are many materials which require heat and pressure as part of their processing in producing certain characteristics. It is common, for example, to place synthetic resin pellets, particles and the like, in a dispersion medium along with various additives in a closed vessel such as an autoclave and to treat the particles by subjecting them to selected heat and pressure conditions in order to expand the plastic pellets into a usable product for molding. While such batch processing has been effective when accomplished with small quantities, the procedure becomes impractical both in terms of space required and cost in large scale commercial production. Batch processing is limited to the contents of the vessel and the limited number of processing cycles or batches which can be conducted in a day. A typical batch requires approximately an hour plus time for loading and unloading. The process is also plagued with problems of pellets clogging the evacuation valve. Further, pressure drop within the vessel during evacuation can result in premature expansion of the pellet within the vessel. This can cause a jam and a melted mass of the contents at worst, and an uneven size and quality of the expanded pellet at best. The pressure within the vessel typically is in excess of 600 psi and the temperature approximately 300 F. In view of the above, enlarging the vessel is not a practical solution for increasing production. Further, various federal and state regulations limit the size of pressure vessels. Stress of the metal under such heat and pressure cause metal fatigue, cracking and danger of rupture thereby further militating against that approach. Use of a larger vessel also would adversely affect heat distribution and mixing.
A continuous processing system would overcome the production quantity and vessel size problems, however, in order to provide such a system, a number of obstacles have to be overcome. The physical properties of the pellet are such that the pellets separate very quickly from the liquid slurry and cluster together. This tends to jam pipes, valves and pumps instantly. It is, therefore, necessary to keep the pellets evenly mixed in the liquid slurry at all times never allowing the particulate material to gather together and coagulate. The slurry somehow must be fed into a high pressure and high temperature chamber in which the particles are being continuously treated, i.e., in this particular example 600 psi and 300 degrees F. Once in the treating chamber the slurry must be stirred continuously throughout its passage in the chamber and the temperature of the particulate material maintained at a consistent level. Still another problem relates to removing the treated material under controlled conditions without damage to the softened material and in a manner to allow the particulate material to expand.
Other materials treated in high pressure, high temperature conditions pose similar handling problems.